A CA system is vital to a cable/satellite pay-TV broadcaster, and the most important part in a CA system is how to add legal users (paid users) into the system and remove illegal users from the system dynamically. The basic architecture of a CA system is EMM (entitlement management message), ECM (entitlement control message), CW (control word), Stream. As shown in FIG. 1, an operator (transmitting side) broadcasts EMM and ECM to each legal user through network broadcasting, the EMM contains a message to be transferred to each user (receiving side) and this message contains an entitlement key (i.e. media key, MK) needed by the user. Each user's device filters the EMM when receiving it, and upon obtaining the message to be transferred to the user in the EMM, the message is decrypted by using a user key (distributed to the user by way of smart card or other means) obtained in advance from the operator so as to obtain the entitlement key therein, then the entitlement key is used to decrypt ECM to obtain CW that is used to encrypt video streams (e.g. MPEG-2). Thus, legal users can watch an encrypted video program by using the entitlement key dynamically distributed to them by the operator, while unpaid users (illegal users) can not watch the encrypted video program due to not obtaining the entitlement key.
In this CA system, EMMs play an important role for distributing the entitlement key. But unfortunately, the length of EMM is quite long in most CA systems. Generally, the length is proportional to the number of users within the CA system, which may grow tremendously in a large system. Because of its length, more bandwidth may be taken to broadcast EMMs, and sometimes, users have to turn on their set-top boxes in order to receive EMMs. Since TS stream of MPEG-2 allows to combine many code streams together, EMM and ECM are transferred together with video streams instead of being transferred through a single channel. Also, EMM varies once a month while ECM varies once per ten seconds, so the bandwidth occupied when they are transmitted can severely influence the receiving and viewing of video programs. This situation causes some services like PPV (Pay Per View), IPPV (Impulsive Pay Per View) and Near-VOD quite inconvenient. For example, in a traditional CA system with 10,000 users, if 1% users (100 users) want to leave the system, the system has to send to each user among the left 9,900 users an EMM containing information of each user, notifying them to change their group and the original entitlement keys they own. Thus it will occupy a large amount of bandwidth to broadcast these notifications, thereby wasting a lot of resources.